What is buffering inputs and outputs

Please someone explain how it's works

Some Drivers as ActiveX mscomm32.ocx do it for application.
It could be so, that application can't follow input-string to serial-port itself, so driver puts string to buffer for application. One ActiveX-property is string lenght (LEN) and maybe Event to application that input buffer size have changed.
Usually Windows application just waits events as button-click etc..

Application can see the string lenght property and read so many bytes from the Input-buffer-string. When application do Input-command, driver push that string to application and clear buffer as many bytes have readed.

Output buffer-lenght property could be set how many bytes must be in the buffer before transfer to serial-port beginning.

I hope this help ?

Are you referring to Analog, Digital or Serial I/O?

You can also buffer analog and digital inputs and outputs. Buffering just means an additional device is placed between the PLC I/O and the end device. The buffer could be a signal conditioner for an analog I/O or an interposing relay for a digital I/O. The buffer could be used to change the voltage or current to the final device or to isolate the I/O.

This information pertains only to Allen Bradley processors.

Older processors, prior to Allen Bradley's ControlLogix Series, updated their inputs and outputs at specific times.
i.e. read inputs, scan program, set outputs, etc.
In ControlLogix processors the I/O are updated asynchronously to the program scan. They can update at specific intervals, which can be set, and also update at a change of state. It is possible for an input to change state during the program scan execution. The point must be “buffered” to prevent this. You can copy the input data from your input tags to another structure and use the data from there.